Regulation is defined as the return of a population to equilibrium density. An operational definition of regulation is convergence to a single density by subpopulations which have been manipulated previously to different densities. The equilibrium density may be fixed or variable. If the equilibrium is variable then regulation may produce instability (numerical inconstancy) and non—density—dependence. Population inertia is the tendency for a population to resist changes away from its current density. If speed of regulation is defined as |s|, the speed of convergence to equilibrium, then inertia is 1/|s|. The evolution of mechanisms of inertia involves changes in the demographic functions, mediated through physiology or behavior, which keep the rate of numerical change low. It is not clear if populations are control systems or non—control systems, which makes the convergence experiment difficult to interpret theorectically. Experiments and observations are needed which will try to distinguish, among stable populations, between those with tight regulation and those with high inertia.